1. Field of the Invention
The invention relates to diode x-ray detectors and more particularly to cadmium telluride based diode x-ray detectors.
2. Prior Art
CdTe has been of interest for the detection of x-rays for several years. It is a particularly attractive material because of its high average atomic number (50) and high density (6.29/cc). In addition it has a large band gap (1.45 eV at 300K) which makes room temperature operation possible.
X-ray detectors based on CdTe offer greater sensitivity and a more compact design than is realized in traditional Xenon gas or scintillator based detectors. In addition, unlike most other solid state detector materials, CdTe detectors can be operated at room temperature. This can significantly decrease the complexity and cost of x-ray detection systems. However, the performance to-date of detectors fabricated in bulk CdTe has been less than ideal, primarily due to poor structural and electrical properties.
The metal-semiconductor-metal (M-S-M) structure has been the most common device configuration for commercial CdTe based, x-ray detectors. The device works on a principle similar to that of a gas arc-discharge x-ray detector. A high voltage is applied which, when incident x-rays create electron-hole pairs facilitating conduction, induces a current flow in the x-ray absorptive semiconductor placed between the metal electrodes. In general, high work function metals have been used in conjunction with high resistivity p-type bulk CdTe. This combination of materials produces low Schottky barriers at the metal-semiconductor interfaces resulting in relatively large leakage currents when a bias voltage is applied to the device. In high flux applications, the problem of leakage current is severe, since it masks currents generated by the incident radiation. In addition, traps present in the CdTe cause the photocurrent to persist after the radiation has been removed leaving an undesirable afterglow.
The problems inherent in the M-S-M structure can be overcome to a great extent through the use of photodiode x-ray detectors which operate in a manner very similar to solar cells. The photodiode x-ray detector involves junction devices, the p-i-n structure being the most appropriate to high energy radiation detection. In such devices, doped regions on either side of the intrinsic region, under the influence of smaller reverse biases than in the M-S-M structures, create a depletion region in which electron-hole pairs can be created by incident x-rays under more exactly controlled conditions. Such devices can be engineered to have the desired temporal response and to exhibit a good linear range of current response.
The advantages of photodiode detectors have been demonstrated by p-i-n device structures fabricated in both Si and Ge. Known devices have generally required cryogenic operation because of smaller band gaps creating too many carriers at room temperature. The CdTe device with its 1.45 eV band gap has accordingly been the subject of continuing investigation because of its theoretical promise for room temperature operation. However, an ideally functioning CdTe p-i-n device has not been readily available due to problems associated with the quality of the intrinsic material, providing effective extrinsic p-type doping, and effecting good ohmic contacts.